LPIC-2 PTR record Practice Question
A company's mail server (Postfix) is rejecting incoming emails from a trusted partner with the error '550 5.7.1 Service unavailable; Client host [203.0.113.50] blocked using zen.spamhaus.org'. The partner's IP is not listed on any public DNSBL. What is the most likely cause?
⚠ Common exam trap
Candidates often take the error message at face value and assume the block is due to a DNSBL listing, but the key is that the IP is not listed. The trap is that Postfix's reject_unknown_client_hostname can display a custom message referencing a DNSBL, leading to confusion.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The partner's SMTP server does not have a valid PTR record for its IP, and Postfix has reject_unknown_client_hostname enabled.
The error message indicates a block using zen.spamhaus.org, but the partner's IP is not listed on any public DNSBL. This rules out options B and C because a DNSBL listing or greylisting would not produce this exact error. Option A (SPF misconfiguration) is unlikely because SPF failures typically produce a different error message (e.g., '550 5.7.1 SPF check failed'). The most plausible cause is option D: the partner's SMTP server lacks a valid PTR record, triggering Postfix's reject_unknown_client_hostname restriction. Postfix can be configured to display a custom rejection message, and the administrator may have set it to reference zen.spamhaus.org as a deterrent. Therefore, the missing PTR record is the most likely cause.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The partner's SPF record is misconfigured, causing Postfix to reject the email.
Why it's wrong here
SPF misconfiguration would cause a different error, not a reference to a DNSBL. The error clearly points to a block related to the client host, likely a reverse DNS issue.
- ✗
The partner's IP is listed on a local DNSBL that is aggregated with zen.spamhaus.org.
Why it's wrong here
The IP is not listed on any public DNSBL, and an aggregated local DNSBL would still result in a similar block but the question states it's not listed on any public DNSBL, so this is not the cause.
- ✗
The mail server is using greylisting and the partner's server has not retried.
Why it's wrong here
Greylisting produces a temporary failure (4xx), not a permanent 550 error, and does not reference a DNSBL.
- ✓
The partner's SMTP server does not have a valid PTR record for its IP, and Postfix has reject_unknown_client_hostname enabled.
Why this is correct
Correct. Missing PTR record triggers reject_unknown_client_hostname, which can be configured to show a custom message like 'blocked using zen.spamhaus.org'. This matches the symptoms exactly.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This LPIC-2 practice question is part of Courseiva's free LPI certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the LPIC-2 exam.